The box seems to be suited for small MOCs where it can be stuck on Lego plates or old-style Technic Bricks. I know that Joey Lock wanted something like this for his Panzer IV Tank MOC, and Sariel's Jeep Wrangler Rubicon has the similarly-sized 8878 PF Rechargeable Battery Box (shown at the left of the last photo above). Using AAA batteries will provide longer run times than a single 9V battery would.

The new battery box is not yet loaded on Bricklink or Peeron, so the PART NUMBER is not known yet! The 7939 set is not yet for sale on Lego.com, so we'll have to wait....

Edited May 14, 2010 by dluders

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Interesting battery box, I hope 6 'AAA' can give the same preformance as 'AA' batteries (sorry, electric cells - they are only 1.5V each) - I surpose if you wish to change from dry cell or rechargable pack and visa versa you can without too much trouble.

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The new battery box was included in new Train sets specifically to lower their prices. The AAA batteries provide the same performance as AA ones, but the general running time of an average motor will be shorter due to their smaller capacity. Then again, the new Train sets include a new motor which is externally identical to the older one, but internally completely changed and PF-system compatible, and it's more energy-efficient than the older one. From what I know being a member of the LEGO's PF group, the new Trains motor provides higher torque than the older one, and it should run just as long on AAA batteries as the older one did on AA ones.

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But you guys must know the lithium polimer battery loose capacity quite fast (like in some years). But in this new box you can place any batterys incuding rechargable ones.

In my experience all rechargeables are the same. I personally use rechargeables in almost everything so it doesn't make a huge difference, but I just wanted a chance to get one of those new packs at a retail price. Looks like I might have to pony up the big bucks...

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According to http://shop.lego.com/ByTheme/Product.aspx?...cn=587&d=70 , the 8878 Power Functions (PF) Rechargable Battery Box only has an output voltage of 7.4V (instead of 9V). So, running a PF Medium or XL Motor with it would not be as good as using 6 ea. AAA batteries in the new PF Battery Box.

8878 costs US$50, and it needs the US$25 8887 10VDC Tranformer to charge it up. Let's assume that the new PF Battery Box will cost US$10. One can buy a LOT of AAA batteries to equal the US$65 difference in cost between the two setups.

Edited May 15, 2010 by dluders

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So, running a PF Medium or XL Motor with it would not be as good as using 6 ea. AAA batteries in the new PF Battery Box.

This is only theoretically true. The power output from standard AA or AAA batteries drops rapidly over the time, especially with high-current-consuming XL motors. Basically, the difference you've shown is true only for fresh out-of-the-box batteries. The rechargeable AA or AAA batteries are much less prone to this effect, and the 8878 battery provides practically constant output until it runs out dry.

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Of course this power difference will only be noted when standard (non-rechargeable) 1.5V AAA batteries are used. Rechargeable NiMH AAA batteries have a voltage of only 1.2V yielding a total of 7.2V, even less than the 7.4V of the 8878 LiPo battery.

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Philo measured the actual (not theoretical) output of the PF Medium and PF XL motors in bench tests. See the bottom of his webpage http://philohome.com/pf/pf.htm :

"PF motors power curves when connected to IR remote receiver -- Since motor driver inside remote receiver has some dropout and current limitation, charts above don’t tell the whole story. The two curves below show the mechanical power of PF motors driven by the remote receiver. These characteristics were measured at 9V (alkaline batteries voltage) and 7.2V (NiMH rechargeable batteries voltage)."

So, the moral of the story is that if you HAVE ROOM for the large PF Battery Box in your MOC, you're better off using AA batteries (because they produce more power). If you have limited space (or want to keep weight down), you can use a small PF battery box but your electrical output will be a lot less.

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So, the moral of the story is that if you HAVE ROOM for the large PF Battery Box in your MOC, you're better off using AA batteries (because they produce more power). If you have limited space (or want to keep weight down), you can use a small PF battery box but your electrical output will be a lot less.

If your model is not remotely controlled or you're simply testing it, the best solution is to use one of the 9V AC power sources, such as the train speed regulator, control center or original RCX, along with a PF adapter cable. Alternatively, you can rig up a 9V transformer to a normal battery box with a little work. As Sariel said, the voltage of alkaline batteries drops to 7-8V after only moderate use, long before they die out. I avoid using batteries whenever possible and always like to run models off wall power instead, even if I have to use long cables to hook them up.